Absorbance Detection

Detection of organic compounds in water analysis

Absorption of light correlates to the energy of a photon that is taken-up by electrons of the substance atom. The electromagnetic energy is transformed into internal energy of the absorbent substance. The absorbance of a substance quantifies how much of the incident light is absorbed by it (instead of being reflected or refracted). Precise measurements of the absorbance at many wavelengths allow the identification of a substance via absorption spectroscopy, where a sample is illuminated from one side, and the intensity of the light that exits from the sample in every direction is measured (see Fig. 1). A few examples of absorption are ultraviolet–visible (UV-Vis) spectroscopy or infrared (IR) spectroscopy.

Fig 1. Concept of absorbance spectroscopy using white light and optical components to filter out light of a specific wavelength that interacts with molecules in the solution. Absorbance at this specific wavelength by the molecules in the solutions is detected as a decrease in light intensity (Spectrophotometer-Source: http://chemwiki.ucdavis.edu/).

Absorption is the amount of light that a substance takes in and does not allow to pass through it. Spectrophotometers actually measure transmission, the amount of light that passes through a sample, but this is converted into absorption by comparing the bulb output to the light that has passed through the sample. Light sources that can be used for absorbance spectroscopy depend strongly on the used substance to label a specific molecule and can span the entire electromagnetic spectrum of light.

Industrial applications that cover the UV spectrum for protein detection is linked to food analysis (for example, characterizing the grading of olive oil as extra virgin, virgin oil, etc., as set out by European regulations) or quality control in the pharmaceutical industry. In addition, industrial applications of absorbance spectrophotometry cover the characterization of water purity or waste water analysis, in addition to the determination of specific organic molecules like nitrate, nitrite or phosphor. This last application is interesting when using the visible light (Vis) spectrum.

Biological application spans also the measurement of the light scatter at 600 nm to monitor the growth rate of a cultured bacterial population and to identify the peak concentration.

Vis absorbance spectrophotometry using monochromatic light sources, like high-power LED, are used in WPI’s LEDSpec spectrophotometer, as 2 or 4 channel system, using up to seven wavelengths. The LEDSpec provides high sensitive and stable absorbance measurement with low-noise characteristics for a specific wavelength. Moreover, high-power LEDs provide a stable light source, no warm-up times, are low-cost and offer a long life.

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